By Daphne Psaledakis WASHINGTON (Reuters) – The United States has imposed several rafts of sanctions on Russia over its invasion of Ukraine in February, targeting its central bank, major lenders, oligarchs and Russian President Vladimir Putin. On Wednesday, Washington warned that the United States would take measures against Moscow in the coming days over “sham” referendums held by Russia in occupied regions of Ukraine. Here are some ways in which the United States could further increase sanctions on Russia: SANCTIONS ENFORCEMENT Experts said they expect Washington will continue to take measur…